I have to admit, I've wondered about this one for years. I recall stumbling onto the Microsoft resource and wondered "how do they get away with this"? What I mean is that commercial data provider terraserver.com is now finally taking Microsoft to court over their imagery project trademark. Should be interesting to see how this one pans out - most likely the big guy will prevail in court (don't they always??)
TerraServer.com is a commerical imagery provider. Users can subscribe to their services and receive updates. Data can also be downloaded and posters and prints are also for sale. The site has a strong consumer focus (ie. want to buy a print of a celebrity's home?) but there's also loads of imagery available for a GIS base (ie. data from Pictometry and others). FYI, TerraServer was founded in 1997 in Raleigh, North Carolina with the simple purpose of finding a commercial market for satellite imagery and aerial photography.
Microsoft's resource (TerraServer USA) has been developed with a partnership in place with the USGS. Users can easily access topos and aerial photos. An online viewer enables you to easily browse USGS quads and then download for use. Personally, I don't know of many users or orgs who depend on this site for their data.
Both sites serve a purpose, however, I'd go to some of the many other free data providers before these one (see http://gisdata.blogspot.com). There's also USGS seamless data distn (http://seamless.usgs.gov/) or my favorite, the USDA Geospatial data gateway